libmsc: use SMPP34_DELIVERY_RECEIPT_* in libsmpp34

Use new definitions in libsmpp34 to set the registered_delivery field
accordingly, as provided by I5b3afff1b3b77cccd949e0606914c7ac3ba6114c.

Moreover, do not set this header field to zero if status reports are
off, the deliver_t structure has been already zeroed so this not
required.

Change-Id: Ie78e17323796120f576b9c0e1bc5ccc32da8ee12